ci: rename FORCE_KERNEL_TAG to EXTERNAL_KERNEL_TAG

FORCE_KERNEL_TAG allows testing kernel uprevs without rebuilding
containers by supplying an external kernel directly for booting on
hardware devices. Renaming it to EXTERNAL_KERNEL_TAG clarifies its
purpose, distinguishing it from KERNEL_TAG which rebuilds containers.

Signed-off-by: Vignesh Raman <vignesh.raman@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31795>

.gitlab-ci.yml

@@ -101,7 +101,7 @@ default:
   before_script:
     - |
       if [ -z "${KERNEL_IMAGE_BASE:-}" ]; then
-        export KERNEL_IMAGE_BASE="https://${S3_HOST}/${S3_KERNEL_BUCKET}/${KERNEL_REPO}/${FORCE_KERNEL_TAG:-$KERNEL_TAG}"
+        export KERNEL_IMAGE_BASE="https://${S3_HOST}/${S3_KERNEL_BUCKET}/${KERNEL_REPO}/${EXTERNAL_KERNEL_TAG:-$KERNEL_TAG}"
       fi
     - >
       export SCRIPTS_DIR=$(mktemp -d) &&